The present invention relates to a photovoltaic element which generates an electric power by incidence of light, and a manufacturing method thereof.
FIG. 1 shows a cross sectional view showing a structure of a conventional photovoltaic element having a hetero junction in which a crystalline semiconductor and an amorphous semiconductor are used. Referring to FIG. 1, the reference numeral 1 represents an n-type crystalline silicon substrate made of a crystalline semiconductor such as single crystal silicon or polycrystal silicon. An i-type amorphous silicon layer 2 and a p-type amorphous silicon layer 3 are laminated in this order on one principal plane (front surface) of the crystalline silicon substrate 1. Further, a transparent conductive film 4 made of, for example, ITO and a comb-like collecting electrode 5 made of Ag are formed thereon. An i-type amorphous silicon layer 6 and an n-type amorphous silicon layer 7 are laminated in this order on the other principal plane (back surface) of the crystalline silicon substrate 1. Further, a transparent conductive film 8 made of, for example, ITO and a comb-like collecting electrode 9 made of Ag are formed thereon.
In practical use as a solar cell, a modular structure is adopted in which numerous photovoltaic elements having such a structure are connected in series via tabs soldered onto the collecting electrodes 5, 9.
The conventional photovoltaic element is manufactured according to the following procedure. First, by means of the plasma CVD method, the i-type amorphous silicon layer 2 and the p-type amorphous silicon layer 3 are successively formed on one principal plane of the crystalline silicon substrate 1, and also the i-type amorphous silicon layer 6 and the n-type amorphous silicon layer 7 are successively formed on the other principal plane of the crystalline silicon substrate 1. Then, by means of the sputtering method, the transparent conductive film 4 and the transparent conductive film 8 are formed on the amorphous silicon layer 3 and on the amorphous silicon layer 7, respectively, and further, by means of the screen printing method, the comb-like collecting electrodes 5, 9 are formed on the transparent conductive film 4 and on the transparent conductive film 8, respectively.
In the photovoltaic element having such a structure, since each of the constitutions other than the crystalline silicon substrate 1 can be formed at a temperature below 200.degree. C. by means of the plasma CVD method, the sputtering method, the screen printing method, or the like, the substrate can be prevented from warping and the manufacturing costs can be reduced. Further, the photovoltaic element having such a structure is manufactured in a low temperature environment in order to inhibit heat damages to the amorphous silicon layers 2, 3, 6, 7, so that a paste of low temperature cure type is used as an Ag paste for the collecting electrodes 5, 9. Accordingly, the electric resistance of the collecting electrodes is high.
In the conventional photovoltaic element as mentioned above, since the Ag paste used for the collecting electrodes 5, 9 is of low temperature cure type, the range of conditions for soldering the collecting electrodes to connect the photovoltaic elements in series is narrow, so that management of the conditions is difficult, solderability is poor, and there is a great possibility of insufficient soldering because silver may be taken solder or a migration may take place by the soldering process.
FIGS. 2A and 2B are model views showing a screen printing process using a conductive paste (Ag paste) utilized for forming the collecting electrodes 5, 9 in a method for manufacturing the above-mentioned conventional photovoltaic element. FIG. 2A shows a view during the process and FIG. 2B shows a shape of the electrode when the process is finished. An emulsion 21 and a mesh 22 are integrated to form a screen mesh 27, and a bedding body 23 is covered with the screen mesh 27 in which a portion of the emulsion 21 is removed in correspondence with a position of an electrode to be formed. Then, a conductive paste 25 is applied onto the bedding body 23 by moving a squeegee 24 to form an electrode 26 having a predetermined width.
In such a screen printing process, 40 .mu.m is an upper limit of the thickness of the electrode with respect to a line width of 120 .mu.m by one printing process, and also there will be a great variation of thickness. Further, a large unevenness of the electrode 26 occurs owing to a patterning shape of the mesh 22, as shown in FIG. 2B. Because of these reasons, the electric resistance will be high and the loss of electric current will be large, thereby giving rise to some of the factors that inhibit improvement of photoelectric conversion properties.